Molecules heat of formation

Lias, S.G. Liebman, J.F. Levin, R.D. Evaluated Gas Phase Basicities and Proton Affinities of Molecules Heats of Formation of Protonated Molecules. J. Phys. Chem. Ref Data 1984, 75,695-808. 

Lias, S. G., Liebman, J. E, and Levin, R. D. (1984). Evaluated gas phase basicities and proton affinities of molecules heats of formation of protonated molecules.. Phys. Chem. Ref. Data 13, 695-808. 

A modification of G2 by Pople and co-workers was deemed sufficiently comprehensive tliat it is known simply as G3, and its steps are also outlined in Table 7.6. G3 is more accurate titan G2, witli an error for the 148-molecule heat-of-formation test set of 0.9 kcal mol . It is also more efficient, typically being about twice as fast. A particular improvement of G3 over G2 is associated with improved basis sets for tlie third-row nontransition elements (Curtiss et al. 2001). As with G2, a number of minor to major variations of G3 have been proposed to either improve its efficiency or increase its accuracy over a smaller subset of chemical space, e.g., the G3-RAD method of Henry, Sullivan, and Radom (2003) for particular application to radical thermochemistry, the G3(MP2) model of Curtiss et al. (1999), which reduces computational cost by computing basis-set-extension corrections at the MP2 level instead of the MP4 level, and the G3B3 model of Baboul et al. (1999), which employs B3LYP structures and frequencies. 

A modification of G2 by Pople and co-workers was deemed sufficiently comprehensive that it is known simply as G3, and its steps are also outlined in Table 7.5. G3 is more accurate than G2, with an error for the 148-molecule heat-of-formation test set of 0.9 kcal mol-1. It is also more efficient, typically being about twice as fast. 

Molecule Heat of formation kcals Number of H H interactions Number of ionic structures... 

The result from a HF-SCF-LCAO computation includes information on the equilibrium geometry of the molecule in addition to thermodynamic information such as total energy of the molecule, heats of formation, and bond dissociation energy. The results of some HF-SCF-LCAO computations are shown in Tables 9-i and 9-5. 

Investigations to find such additive constituent properties of molecules go back to the 1920s and 1930s with work by Fajans [6] and others. In the 1940s and 1950s lhe focus had shifted to the estimation of thermodynamic properties of molecules such as heat of formation, AHf, entropy S°, and heat capacity, C°. 

Until now, we have discussed the use of additivity schemes to estimate global properties of a molecule such as its mean molecular polarizability, its heat of formation, or its average binding energy to a protein receptor. 

MINDO/3, MNDO, and AM 1 wxrc developed by the Dervar group at the University of i exasat Austin. This group ehose many parameters, such as heats of formation and geometries of sample molecules, to reproduce experimental quantities. The Dewar methods yield results that are closer to experiment than the CN DO and IN DO methods. 

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

You have seen that measurements of heats of reaction such as heats of combustion can pro vide quantitative information concerning the relative stability of constitutional isomers (Section 2 18) and stereoisomers (Section 3 11) The box in Section 2 18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AH ) for many molecules The following material shows how two different sources of thermo chemical information heats of formation and bond dissociation energies (see Table 4 3) can reveal whether a particular reaction is exothermic or en dothermic and by how much... 

The following data (Table 1) for molecules, including hydrocarbons, strained ring systems, molecules with heteroatoms, radicals, and ions comes from a review by Stewart.For most organic molecules, AMI reports heats of formation accurate to within a few kilocalories per mol. For some molecules (particularly inorganic compounds with several halogens, such asperchloryl fluoride, even the best semi-empirical method fails completely. 

MINDO/3 is the earliest of the Dewar methods. It provides more accurate geometries and heats of formation than CNDO or INDO, and has been used widely. The limitations of the INDO approximation, on which MINDO/3 is based, frequently lead to problems of accuracy when dealing with molecules containing heteroatoms. 

Many problems with MNDO involve cases where the NDO approximation electron-electron repulsion is most important. AMI is an improvement over MNDO, even though it uses the same basic approximation. It is generally the most accurate semi-empirical method in HyperChem and is the method of choice for most problems. Altering part of the theoretical framework (the function describing repulsion between atomic cores) and assigning new parameters improves the performance of AMI. It deals with hydrogen bonds properly, produces accurate predictions of activation barriers for many reactions, and predicts heats of formation of molecules with an error that is about 40 percent smaller than with MNDO. 

Resonance stabilization energies are generally assessed from thermodynamic data. If we define to be the resonance stabilization energy of species i, then the heat of formation of that species will be less by an amount ej than for an otherwise equivalent molecule without resonance. Likewise, the AH for a reaction which is influenced by resonance effects is less by an amount Ae (A is the usual difference products minus reactants) than the AH for a reaction which is otherwise identical except for resonance effects ... 

The tnhahdes of phosphoms usually are obtained by direct halogenation under controlled conditions, eg, in carbon disulfide solution in the case of the triiodide. Phosphoms trifluoride [7647-19-0] is best made by transhalogenation of PCl using AsF or Cap2. AH of the phosphoms tnhahdes are both Lewis bases and acids. The phosphoms tnhahdes rapidly hydroly2e in water and are volatile. Examination by electron diffraction has confirmed pyramidal stmctures for the gaseous tnhahde molecules (36). Physical properties and heat of formation of some phosphoms hahdes are hsted in Table 7. 

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